Background Information
The section of the River Dun at Dunbridge, Hampshire, was subject to habitat restoration project as part of the Flow Resilient Sustainable Habitat ( FReSH ) Water Programme. The project was funded by Southern Water and delivered through the Wessex Rivers Trust.
Scope of the Project
The river exhibits a gravel-rich bed, diverse marginal fringe, and benefits from a good ratio of direct sunlight to dappled shade. However, the channel profile through much of the upper-mid reach of the site is modified and overly uniform. Historic dredging has resulted in a significantly over-deep channel, resulting in widespread and uniform sedimentation and a macrophyte community dominated by species adapted to slower flows. The project was designed to increase sinuosity, and diversity of channel width and flow patterns, improve habitat resilience during low flows, macrophyte abundance, and salmonid spawning/nursery habitat.
Our Solution
Large woody material and coarse woody material features were introduced to the river channel in the form of hinged and staked bankside trees and brash berms. To save costs all woody material was sourced in the immediate locality of each feature. The brash berms and hinged trees narrowed the channel to approximately 2/3 existing width to increase water velocity and prevent sedimentation and increase sinuosity. A long reach excavator was used to introduce approximately 500 tonnes of 20 40mm washed gravel and flint gravel rejects into the channel. This raised the whole bed over approximately 100m to create a more natural channel profile and gradient, increasing f low speeds for the benefit of favourable chalk stream macrophytes and wild salmonids. Due to the presence of water vole burrows in the bank an excavator with sufficient reach to maintain a 5m stand off distance from the water’s edge was used Gravels w ere all positioned by excavator from the left hand bank due to lack of access to the right hand bank. Gravels were then shaped by hand to achieve a subtle and naturalistic pool riffle sequence through the reach.
